Boat hull



1961 c. HAMLIN 2,997,974

BOAT HULL Filed Sept. 22, 1959 2 Sheets-Sheet 1 Air weight 10f wlndwurd Air weight of Buoyancy leeward keel of leeward keel FIG.5

INVENTOR. Cyrus Hamlin /IMEKL ATTORNEY 9, 1961 c. HAMLIN 2,997,974

BOAT HULL Filed p 22. 1959 2 Sheets-Sheet 2 INVENTOR. Cyrus Hamlin ATTORNEY 2,997,974 BOAT HULL Cyrus Hamlin, Manset, Hancock County, Mame Filed Sept. 22, 1959, Ser. No. 841,491 11 Claims. (Cl. 114-142) This invention relates to a boat hull comprising a main hull and a pair of hollow bilge keels widely spaced apart athwartships of the main hull and adapted to be filled with and emptied of water ballast and to retain said water ballast at the option of the master. While my invention may be advantageously used in both power and wind driven craft, it is particularly useful when applied to sailing craft, the ballasted bilge keels providing both unusual stability and the lateral resistance necessary in windward sailing which is conventionally provided by deep keels, centerboards or bilge boards.

The bilge keels are advantageously placed as far apart athwartships as the form of the hull will allow and are of such size and so located that, when filled with water, they will provide a righting moment even when the boat is heeled 90 from the vertical. The righting effect of the water ballast may, when desired, be supplemented by iron shoes on the bottoms of the bilge keels, as described below. The bilge keels may be applied either to a roundbottom or hard-chine main hull and may readily be fabricated of wood, plywood, sheet metal, plastic or other material, and applied to a hull of the same or a different material.

Preferably the bilge keels are so attached to the main hull that they may be removed or replaced for repair or other purposes Without disturbing the main hull structure.

The bilge keels when filled with water provide adequate ballast for sailing; when emptied, the result is a boat lightened in gross weight by 15 or 25 percent or more even up to 50 percent. This feature has great advantages in overland trailing, since it not only greatly facilitates handling the boat in launching and hauling, but also substantially reduces the cost of a trailer adequate to handle the boat.

While there is no limit to the size of boat to which my bilge keels may be applied, they are particularly adapted for use on sailboats in the 18 to 30 foot overall length range. This is the range of ballasted sailboat length popular for overland trailing.

Aside from the advantage in trailing of being able to jettison water ballast, other advantages of my invention are realized in both trailing and non-trailing boats in a great range of sizes.

I am aware that water ballast in lieu of discharged cargo weight has been used for many years in steam vessels and that bilge keels and bilge boards have long been used in sailboats. But as far as I am aware the particular combination of features of my present invention with its attendant advantages has never before been realized and is completely novel.

While my invention is particularly adapted to sailboat hulls, many of its advantages may be realized in powerboat hulls. In particular it has the property of increasing the period of roll without decreasing the fundamental stability. This favorable result is due to two factors: (1) The increase in angular inertia due to the wide athwart ships spread of the ballast in the bilge keels; and (2) the continuous dynamic damping effect on the roll resulting from the increasing effective weight of the up-moving keel and the decreasing effective weight of the down-moving keel as they move out of and into the water.

In the following description and in the drawings a preferred embodiment of my invention will be illustrated as applied to a small cruising sailboat of approximately 21 feet l.o.a., but this description is intended to be exemplary States atent O Patented Aug. 29, 1961 only and not to limit my invention, the scope of which is defined in the appended claims.

In the drawings FIG. 1 is a sail plan and view of the hull above the water of a boat embodying my invention;

FIG. 2 is a cross section showing the location of my bilge keels-on the right to a round-bottom hull and on the left to a hard-chine hull;

FIG. 3 is a side view of a hard-chine hull;

FIG. 4 is a plan of a hull;

FIG. 5 is a transverse section of a round-bottom hull at heel;

FIG. 6 is a transverse section of the same hull at 20 heel;

FIG. 7 is a transverse section of a conventional hull at 20 heel.

FIG. 8 is a construction detail showing my bilge keel applied to a round-bottom hull;

FIG. 9 is a similar view showing application to a hardchine hull; and

FIG. 10 is a perspective view, partly in section showing one method of construction a bilge keel according to my invention.

In the drawings the hull 1 has a conventional bow 2 and stern 3 on which a rudder 4 is conventionally hung. It is provided with a foredeck 5, afterdeck 6, cabin trunk 7 and coaming 8. A mast 9 is stepped on the cabin roof and stayed by a forestay 10 and shrouds 11. The sails may comprise a mainsail 12 and a jib 13. A cockpit or standing room 14 is provided aft of the cabin trunk. The construction and arrangement of the upper part of the boat may thus be conventional for boats of this general type.

If the hull is of the hard-chine type, the topsides 15 will be straight or nearly so in cross section and the bottom 16 may be either straight or slightly curved. Topsides and bottom will meet in a chine 17 My hull, however, preferably differs from a conventional chine hull in that a step 18 is provided inboard from the chine for the attachment of a bilge keel 19. While this is a preferable hull form, the step is not essential since the bilge keel may be attached to the bottom without the necessity of a step.

If the hull is of the round-bottom type, topsides 20 and bilge 21 preferably form a curve continuous in cross section (FIG. 2, right side).

Each bilge keel is provided with a fill hole and plug 24-, and a drain hole and plug 25 (FIG. 3). The fill holes instead of being in the sides of the keel may be connected by pipes 24' to deck plates (FIG. 6). Pipe 24' may, if desired, extend to the bottom of the bilge keel and be provided width a conventional pump plunger 25' and rod so that the bilge keel may be emptied without hauling the boat out of the water. In this case an air vent 24" is provided to permit filling the keel through pipe 24'. The drain hole fitting is advantageously placed at the after lower corner of each keel to reduce water resistance and ensure drainage when the boat is hauled out on a sloping beach or ramp.

As shown in FIGS. 4 and 10 I have found it desirable but not necessary to build the bilge keels in airfoil shape with relatively flat outboard surfaces 26. This provides better lateral lift and produces a more pronounced wave system on the inboard sides, thus giving greater lift to the stem.

The volume of the keelsis preferably such that the total weight of the water ballast in them is of the order of twenty to twenty-five percent or even up to fifty percent of the total loaded weight of the boat. They are advantageously provided with iron or similar shoes 28 to absorb wear in grounding as well as to provide a small amount of fixed ballast.

In FIGS. -7 a comparison is shown of the stability of a conventional keel boat and of a boat built according to my invention at diiferent angles of heel.

FIGS. 5-7, CG indicates center of gravity of the Whole boat; CGK indicates center of gravity of the keel; CB indicates center of buoyancy; and RA indicates righting arm. The arrowed lines indicate by length and direction the forces acting on the hull as explained by the appended legends.

"in any boat the stability is defined as the displacement (weight) times the righting arm or horizontal distance between center of gravity and center of buoyancy. In the conventional keel sailboat the righting arm, RA (FIG. 7), is increased by lowering the center of gravity in'the hull by means of ballast, such as lead or iron fastened to the outside of the keel as at 23', FIG. 7. The stabilizing efiect of the ballast in the conventional sailboat at normal angles of heel is the air weight of the ballast material less the buoyancy of the Water it displaces times the righting arm of the ballast. The latter is fairly short at normal angles of heel (FIG. 7).

In FIGS. 1-3 and 57 LWL indicates the waterline In my invention the windward bilge keel has an efiective weight equal to its air weight as soon as the keel is clear of the water; thus a pound of a less dense material such as water is more useful for stability than a pound of a very densematerial such as lead immersed in water as in the conventional keel. In addition this more effective ballast reaches its maximum righting arm at the normal angles of heel rather than at 96 heel as is the case with the conventional keel sailboat. On the other hand, as soon as the leeward bilge keel is fully submerged, preferably as the normal heel angle is approached, it neither adds to nor .substracts from stability, since the weight of the iron shoe, structure, and contained Water is very nearlyequal to the buoyancy of the displaced water, i.e. the specific gravity is about 1.0.

Thus at normal angles of heel, e..g. 2030, my invention has two. advantages with respect to stability over conventional keel boats: (l) The increased length of the righting arm, and (2) the loss of buoyancy of the wind Ward keel at it rises out of water.

A major object of ballasting a sailboat is to prevent capsizing; it is normally assumed that a boat which will right itself from a heel angle of 90, i.e. on its beam ends, is non-capsizable. In my invention this require ment is met by the design of the hull so that the center of buoyancy is close to the deck at 90-of heel and, if necessary, adjusting the size of the metal shoes on the bilge keels to locate the CG relative to the CB at 90 heel to provide a positive righting couple.

In power boats the factors described above that tend to increase the stability of a sailboat also have the desirable eliect of damping rolling and thus lengthening the period of roll without decreasing stability.

The improved stability of my bilge keels as compared with an equivalent metal keel is apparent from these figures.

FIG. 8 shows in cross section one method of attaching my bilge keels to a round-bottom boat without a step in its sides. The hull is sheathed with nailed and glued strip planking 29. The sides 30 of the bilge keel 31 are of marine plywood glued and screwed to a longitudinal block 32. A longitudinal top member 33 whose upper surface 34 is dressed to conform to the hull contour is glued to the outside of the hull, and inside the hull a longitudinal pad 35, somewhat longer and wider than member 33, is glued. These parts are of suitable woods for marine use such as oak, mahognay, cedar and the like.

Two longitudinal rows of bolts or studs 36 and 37 pass diagonally through pad 35, hull planking 29, top member 33 and block 32, securing the bilge keel 31 firmly to the hull. Pad 35 distributes the strain of the bilge keel effectively fore and aft and athwartships of the main hull.

At the same time by removing bolts 36 and 37, a relatively simple operation, the bilge keel may be removed when necessary without disturbing the main hull structure or interior joiner work.

A functionally similar but structurally somewhat different method of attachment of my bilge keels to a hardchine boat may be used as shown in FIG. 9 in a cross sectional View. The meeting ofthe hull bottom 38 with the topsides 39 is interrupted by a step 46. Bottom, step and topsides may be made of marine plywood. The framing here consists of side ribs 41, bottom ribs 42', chine log 43, floors 44, step ribs 45 and gussets 46; These are assembled and secured by conventional gluing and metal fastenings such as screws and bolts.

The bilge keel 47 is constructed similarly to bilge keel 31 except as to its shape, the sides 48 being of marine plywood. The bilge keel is advantageously narrower at the bottom than at the top, the bottom being closed by longitudinal block 49, shod with iron 28. Longitudinal upper block 50 is screwed and glued to the sides. The upped surface of this block is dressed to mate with step 40.

The bilge keel is secured to the hull by a longitudinal row of bolts 51, screwed into nuts 52 that are brazed to a strip of bronze angle 53 fastened to the under side of block 50. To absorb the tension of the bolts and distribute the strain of the bilge keel I provide a series of blocks 54, one for each bolt, and a clamp 55 running the'iength of the bilge keel on top of floors 44.

It will be seen that when the bolts are made up, the bilge keel 47 is drawn firmly against the hull and secured there, while by removing the bolts the bilgekeel may readily be removed for repair or other purposewithout disturbing the remainder of the structure.

The more outstanding advantages of my novel hull with water-balast bilge keels may be sumarized as follows:

It has great stability with shoal draft.

Its stability is superior at moderate angles of heel and is sutficient at emergency angles of heel.

As soon as the one bilge keel (the windward keel in a sailboat) rises out of the water, righting moment increases owing to loss of buoyancy of the bilge keel on the high side; simultaneously the wetted hull surface is decreased, thus increasing speed.

The widely spaced bilge keels increase rolling inertia and thus tend to damp out the jerky motions in rough water ordinarily encountered in a small light boat.

The possibility of draining water ballast'from the bilge keels and thus reducing hull weight by 1'5-25 percent or even up to 50 percent is a great advantage in overland trailing since it permits use of a smaller, lighter and less expensive trailer than would be needed for a heavier boat, reduces the fuel cost of towing boat and trailer, and facilitates handling the boat on and off the trailer.

The hull will stand upright when beached.

The bilge keels may readily be removed for replacement or other purposes without disturbing main hull structure or other fittings.

My hull eliminates the necessity for a centerboard with its awkward trunk, or a relatively deep iron or lead keel, or a centerboard-keel combination.

My hull is easily made non-sinkable since the specific gravity of the ballast=(approx.) 1.0. In a wooden hull the buoyancy of the hull structure would provide the necessary flotation; in a hull of metal or fiberglass or other dense material, a very small amount of flotation material, tanks, etc., would provide the same efiect.

My hull, if damaged and filled, can receive additional buoyancy by pumping the water out of the bilge keels; or, by pumping only the leeward keel dry, it can have a. measure of stability for sailing not found in the usual filled boat.

When under sail beforethe Wind or in light airs, the bilge keels may be emptied, the displacement and wetted surface being thus reduced with resultant increase in speed.

My hull, by virtue of its simplified construction and cost-free ballasting material, is less expensive than the conventional keel or centerboard boat.

I claim:

1. A boat hull structure comprising a central main hull having suficient displacement to provide flotation for the boat and a pair of hollow laterally spaced ballast bilge keels rigidly attached to the main hull at such a height that, when the boat is floating on an even keel, an upper edge of each bilge keel is above the Waterline and a major portion of each bilge keel is submerged whereby when the boat heels a previously submerged portion of the bilge keel on the higher side emerges from the water, each bilge keel being adapted to be filled with and emptied of ballast water and to retain such ballast water at all angles of heel of the boat, access means to the interior of each bilge keel adapted to permit filling and emptying each bilge keel and means adapted to prevent loss of ballast water through said access means at all anges of heel.

2. A hull structure as defined in claim 1 in which the bilge keels are structurally independent of the main hull structure and are removable from the main hull without disturbing the main hull structure.

3. A hull structure as defined in claim 1 in which the bilge keels have a water-ballast capacity in the range of 15 to 50 percent of the total hull weight.

4. A hull structure as defined in claim 1 in which the depth of the bilge keels is at least as great as the depth of the main hull, whereby the hull remains upright when beached.

5. A hull structure as defined in claim 1 in which the volume and placement of the bilge keels is such with respect to the center of buoyancy of the hull when heeled at 90 to the vertical that the keels filled with water exert a positive righting moment.

6. A hull structure as defined in claim 1 in which each bilge keel is shod with a metal shoe attached to its lower edge.

7. A hu-ll structure as defined in claim 1 in which the bilge keels are spaced apart a distance substantially equal to the beam of the main hull.

8. A hull structure as defined in claim 1 in which the means for filling each keel with and emptying it of water comprises a discharge pump operable from the deck and an air vent and a fill hole opening through the deck.

9. A sailboat comprising a hull structure, a sail, a mast and a rudder, a rigid main hull having suflicient displacement to provide flotation for the boat, a pair of laterally spaced-apant bilge keels rigidly attached to the main hull, the bilge keels being hollow and adapted to be filled with and retain water ballast, each keel being provided with means for filling it with and emptying it of water, and means for preventing loss of water through said filling and emptying means at extreme angles of heel, an upper edge of each bilge keel being above the waterline and a major portion of each bilge keel being submerged when the boat floats on an even keel.

10. A hull structure as defined in claim 1 in which each bilge keel has a horizontally curved side, the horizontal section of each bilge keel being asymmetric with the greatest curvature on the inboard side.

11. A hull structure as defined in claim 1 in which the bilge keels are so placed that when the bilge keels are filled with water and the hull is heeled to raise the bilge keel on the upper side at least partially out of the water, the emerging bilge keel has a righting arm of a length at least equal to one half the beam of the main hull at its widest point.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Popular Science, vol. 158, No. 6, June 1951 (page 101 relied on). 

